Assuming that you have an XP disk:
Boot from the CD
Let the Setup go through the first part of the Installation procedure.
When you come to the screen in which it says "Welcome to Setup." press Enter to Setup Windows XP.
Press F8 to agree to the End User License.
Let the Setup search your system for previous versions of Windows.
When the Setup is finished searching your system, select your Windows XP Installation and press the R key on your keyboard to start the Repair Procedure.
Setup will delete all major system files and then replace them with new copies.
You will have to download all of the updates from Microsoft Windows Update again because it replaces the files.